Translation of abstract (English)

Chemotaxis enables bacteria to quickly find optimum growth conditions. Sensing attractants or repellents is based on a simple two-component signaltransduction system. The chemotaxis system of Escherichia coli is thoroughly studied and allows cells to move towards attractants and away from repellents. Effectors are sensed by transmembrane receptors, which are organized in polar and lateral clusters. The cluster core is composed of receptors, the histidine kinase CheA and the adaptor protein CheW. All other chemotaxis proteins, like response regulator CheY and its phosphatase CheZ, as well as the adaptation proteins CheR and CheB, localize either to receptors or to CheA. Despite minimal complexity, this system demonstrates amazing performance that remains partly unaccounted for, despite decades of intensive research. Amongst others, there is a lack of data describing the temporal dynamics of involved components. Therefore a focus of this work was to systematically analyze the intracellular mobility of all chemotaxis proteins on receptor clusters in vivo, using FRAP (fluorescence recovery after photobleaching). Here we show that receptors are very firmly integrated in clusters and together with CheA and CheW build a core structure, with which the remaining components associate with lower affinity, with the strength of the interaction being dependent on the respective protein function. We also observed that active clusters are more stable. In contrast, temperature seems to have no effect on cluster stability and we did not observe brachiation of bivalent proteins within chemotaxis clusters. Effective chemotaxis requires adaptation, which enables bacteria to response to changes in concentrations of effectors. The Adaptation system of E. coli comprises two adaptation proteins, CheR and CheB, which regulate receptor activity by modification. While methyltransferase CheR is constitutively active, methylesterase CheB is activated via phosphotransfer from CheA. It was reckoned for many years that this negative feedback mechanism is sufficient for adaptation. However, in this work we could demonstrate that this is not the case. We thus accessed the real function of CheB phosphorylation. Our experiments demonstrate that activation of CheB stabilizes its binding to chemotaxis clusters and mediates robustness against cellular variations of chemotaxis proteins We investigated methylation reactions on receptors and found that clustering accelerates modification about 40%. In heterologous receptor clusters, consisting of both major receptors, we observed crossmethylation of aspartate receptor Tar as a result of stimulation by serine. On the other hand, α-methylaspartate did not cause crossmethylation of serine receptor Tsr. This indicates a weak receptor coupling within receptor clusters, which result in selective methylation of ligand-bound receptors by CheR. These results support a theory in which methylation and demethylation reactions rely on receptor activity. Additionally, the activity-dependent changes in cluster stability mentioned above could play a role in adaptation. This study reveals that dynamic interactions at receptor clusters represent a so far little-noticed level of regulation in chemotaxis of E. coli.